Major mines, ports and other projects are assessed for their future potential impact on nationally important parts of the environment. Photo: Robert Rough

The projects are then approved, rejected or amended. The federal government wants to hand its environmental approval powers to the states and territories where the projects will be built, in changes before the Senate this week.

The government has said this would create a "one-stop shop" for approvals, speeding up the process and saving projects about $426 million in costs.

The Environment Department said most of this – $417 million – related to time that "one-stop shops" would have saved. It calculated this based on the value of 52 major projects that had been assessed for environmental approvals under the current federal scheme in a 2014 report.

An Australia Institute study said on Monday this was exaggerated and based on "notoriously unreliable" estimates of the projects' worth, relying on data from industry lobby groups. Such values could not be assessed accurately without "consultation with companies and specialist analysts".

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Many of the projects also did not proceed despite being granted environmental approval because they were not financially viable, the study, commissioned by the World Wildlife Fund of Australia, said.

"Unsurprisingly, all analysis is skewed to represent the interests of the lobby groups' members rather than the public interest," it said. Few projects had been rejected under environmental regulations: "Changes … should be based on objective assessment of the public interest rather than low-quality economic assessment."

The study also criticised the data lobby groups used to support the savings claim, including the Minerals Council of Australia and the Business Council of Australia.

Co-author Rod Campbell said they had not considered any economic or social benefits of the current scheme: "By omitting that, all of these studies overstate their case for cutting back this regulation."

Mr Campbell defended the study – paid for by a group opposed to the proposal – saying no economist could pretend to be wholly objective: "I stand by all the figures in it. I'd like to see the same [approach by] lobby groups and their commissioned assessments."

All the states and territories have signed draft or final agreements with the federal government to take over final environmental approvals of more major projects. At least two crossbench senators support the change but it is unclear whether the bill has sufficient support to be passed.

Conservationists have said that it would remove federal oversight over environmental approvals, with state governments often having vested interests to approve major projects because they receive royalties from the companies behind them.

This comes amid broader environmental law changes, including removing environmentalists' legal right to challenge environmental approvals of major mining and other projects in the courts.

A spokeswoman for Environment Minister Greg Hunt said the estimated saving was "based on the Australian government method for calculating regulatory costs and has been verified by the Office of Best Practice Regulation".

"These figures are extremely conservative, and based only on the direct cost savings to business. The implications across the Australian economy are much bigger."